Return true instead of the new PID from %background

[jpco]

I believe the fact that %background returns the PID of the process it creates is an oversight.

• It only happens as a side-effect of assigning apid to the new PID.
• The function has not changed since the "assignment returns the value" change went into effect.
• Other shells (including, most relevantly, rc) return true from command &
• It would be consistent with other cases where the "assignment returns the value" change had knock-on effects (see Does anybody here use `es -e`? #70).
• It is entirely redundant to set $apid, print the new PID if interactive, and return the PID
• In general there is a convention in es commands that they either produce output by returning it, or by printing it/performing side effects, and there is a pattern of defining functions (e.g., var) which echo the value returned from "inner" functions (e.g., %var)